The present invention relates to a pressure-sensitive electrically conductive elastomeric composition. More particularly, the invention relates to a pressure-sensitive, electrically conductive elastomeric composition providing elastomeric material thereof which has such a characteristic property that in the normal state where no pressure is applied, the composition is electrically insulating or highly resistive, but under application of a pressure, the composition is electrically conductive or little resistive.
Materials having such pressure-responsive conductivity are known in the art. For example, the specification of U.S. Pat. No. 2,044,080 discloses that when an electrically conductive material such as granulated carbon interposed between electrodes is pressed, the resistance (or resistivity) of the material is reduced. Further, the specification of U.S. Pat. No. 3,806,471 discloses that various semi-conductors may be used for improving characteristics of pressure-responsive materials, that granules can be agglomerated with a binder and that in order to reduce occurrence of the hysteresis phenomenon and prevent wearing of pressure-responsive materials, it is preferred that the shape of granules are uniform spherical or particulate. Still further, the specification of U.S. Pat. No. 3,710,050 discloses that a material comprising an electrically conductive powder and 20 to 50% by volume of a rubber powder has a good pressure-responsive conductivity and it can be used effectively.
In these conventional materials, however, since powders are used as they are in the state without using a dispersant such as a rubber and the like, there is inevitably involved a defect that these pressure-responsive, electrically conductive materials must be contained and stored in special space. Indeed, in the invention of the above-mentioned U.S. Pat. No. 3,806,417, the granules are agglomerated with a binder, but the flexibility of the material is still insufficient. Further, the specification of U.S. Pat. No. 2,305,717 proposes a method in which powders or granules are confined in spaces of cells of foamed bodies. According to this method, however, it is impossible to prevent fall-down of the powders or granules completely. This problem of fall-down of powders may be solved by coating surfaces of cells of foamed bodies as disclosed in the specification of U.S. Pat. No. 3,629,774, but in this case, the range of variation of the electric resistance of the resulting pressure-responsive conductive material is very narrow. More specifically, since the electric resistance in the normal non-pressed state is not sufficiently high and the ratio of the electric resistance in the normal nonpressed state to the electric resistance under application of a pressure is low, the material is not suitable for switch elements and the like.
It is known that some of electrically conductive composition comprising an electrically conductive powder such as metal, carbon and semi-conductor incorporated in an elastic material such as a rubber show some amounts of pressure conductive effect. The material of this type is excellent and advantageous over the above-mentioned powdery material or the powder or granule confined in a foamed body in the point that the material is flexible and has rubbery characteristics. However, with the material of this type, the object is basically not to attain a pressure-responsive electric conductivity. It has heretofore been conceived that it is a fatal defect involved in materials of the type comprising furnace black or acetylene black incorporated in an elastomer that variation of the resistance (not only by the change in the configuration but mainly by the change in the volume resistivity) is caused by compression or elongation imposed on the materials. As an invention utilizing this pressure-responsive electric conductivity, there can be mentioned, for example, a material disclosed in the specification of U.S. Pat. No. 3,801,839. This material is, however, defective in that the electric conductivity in the normal open state is high and the ratio of the change of the resistivity (sometimes referred to as "sensitivity" in this specification) is low. More specifically, if such pressure-responsive, electrically conductive rubber is used for a switch element, it is desired that the ratio of the resistivity in the non-pressed open state to the resistivity in the state compressed under application of a pressure is at least 10, preferably at least 100. In the material of the above-mentioned type, the compressive force necessary for realizing such conspicuous change of the resistivity is much higher than the level not causing irreversible changes in the strength and other properties of the material.
Pressure-responsive, electrically conductive materials including relatively coarse particles of carbon or metal are disclosed in, for example, the specifications of U.S. Pat. Nos. 2,951,817; 3,509,296; 3,578,733 and 3,952,352. Similar materials are disclosed in the specifications of U.S. Pat. Nos. 3,760,342 and 3,883,213, though these materials are not used in the field where pressurizing and releasing are repeated. When a filler having a particle size of 2.5 to 25 .mu.m is used in these conventional materials, good characteristics can be expected in the initial stage of actual application. However, these materials are still insuficient in the durability of the pressure-responsive characteristic. More specifically, in case of switch elements, it is generally required that the operation should be repeated at least one million times, but materials of this type can endure the operation repeated only several hundred thousand times at highest. Similarly in case of a material including a fine metallic powder as disclosed in Japanese Patent Application Laid-Open Specification No. 74-114798, the durability is still insufficient.